This invention relates to a measuring instrument for measuring and displaying signals, comprising a display system controlled by a time base circuit for displaying a signal to be applied to the measuring instrument under the control of the time base circuit which comprises a digitally adjustable integrator for generating sawtooth voltages with different slopes for different operational states of the display system.
The invention also relates to a time base circuit suitable for use in such a measuring instrument.
Such a measuring instrument is, for example, an oscilloscope in which the display system may be a cathode ray tube on which signal waveforms can be displayed. The oscilloscope is then supplied with a signal, for example, a periodic signal, which is applied on the one hand to the vertical deflection plates of the cathode ray tube via a vertical deflection system, and which is supplied on the other hand via a horizontal deflection system comprising the time base circuit. The time base circuit is periodically triggered in response to the signal so as to generate a sawtooth-shaped signal which is fed to the horizontal deflection system.
Such a measuring instrument is known from European Patent Application EP 0 113 975 A2. which corresponds to U.S. Pat. No. 4,705,961 Nov. 10, 1987. The time base circuit described therein comprises a digitally adjustable integrator, i.e. the current carried by an integration capacitor is digitally adjustable. An embodiment of such a digitally adjustable integrator is shown in FIGS. 3 and 4 of EP 0 113 975 and described in more detail on page 5, line 1 to page 8, line 26 of said Application. As is described on page 5, line 38 to page 6, line 9 thereof, the integration current may be supplied to different integration capacitors via a switch as a function of the desired time base setting, and the integration current itself may be digitally adjusted, as is described on page 7, lines 8-16 and page 8, lines 20-23 of EP 0 113 975 A2. FIG. 7 of said document in addition shows an electronic swich-over of, among other items, the integration capacitors. To have available an accurately calibrated time base for various time base settings, the time base should be calibrated, which may be done manually in that a known signal is applied to an input of the measuring instrument and it is visually checked for each time base setting whether the display system gives a desired response. The setting of the measuring instrument may be changed in the case of an incorrect response until a desired response takes place. Such a time consuming calibration procedure must be repeated in the case of changes of, for example, system characteristics of the measuring instruments if accurate measurements are to be insured.